1. Field of the Invention
The present invention relates to a starter for starting up an internal combustion engine, and more particularly to such a starter which is equipped with an overheat protection device.
2. Description of the Related Art
In general, at a start of a motor vehicle engine, the user manually controls operation of a starter through manipulation of a key switch. In this instance, if a return failure of the start switch or a like abnormal condition occurs, an armature coil may undergo dielectric breakdown to cause a short-circuit between coils whereupon a large current of several hundreds amperes is continuously supplied from a power source to a starter motor for a long time, thus exerting an excessively large thermal load onto the starter. Alternatively, if for some reason a failure occurs with an electromagnetic switch, the starter motor may be continuously energized under a no-load condition.
To cope with abnormal conditions involving excessively large thermal loads, it may be considered that the starter motor be electrically separated from the power source by using some means. Typical examples of such prior considerations are disclosed in Japanese Utility Model Laid-open Publication (JP-UM-A) No. 04-64972, International Publication No. WO 00/19091, Japanese Patent Laid-open Publication (JP-A) No. 10-66311, and French Patent Laid-open Publication (FR-A) No. 2785086.
In a starter disclosed in JP-UM-A 04-64972, a brush lead wire (brush pigtail) has a portion with reduced cross-section to provide a fuse function so that when a starter motor is continuously energized for a long time, the portion of reduced cross-section undergoes melting or fusion to cut down an electric circuit including the brush lead wire. However, since the brush lead wire is made of copper, the melting temperature of the bush lead wire is above 1,100° C. Furthermore, owing to good thermal conductivity of copper, the brush lead wire made of copper, even after melting, is able to transmit high temperature to and eventually damage neighboring components. This poses a problem in terms of safety.
Furthermore, in the case of a starter using a permanent magnet in a field system of an electric motor, since, as shown in FIG. 20 hereof, a brush lead wire 200 is directly connected or otherwise welded to a motor lead wire 210, a joint portion between these lead wires 200, 210 is disposed near a grommet 220 in which the motor lead wire 210 is supported. In this arrangement, if the brush lead wire 200 is configured to have a fuse function, the grommet 220 may be damaged under the effect of a high temperature produced when the brush lead wire 200 is melting.
For the brush lead wire, the motor lead wire or a like wire which is formed by a number of fine conductors bundled together it is very difficult to form a weld connection between these wires because of a large contact resistance between adjacent conductors. Thus, the brazing is used in place of the welding. The brazing is, however, relatively low in productivity as compared to the welding and hence increases the production cost.
A starter disclosed in WO 00/19091 has a thermal fuse incorporated in a motor circuit such that when heated at a predetermined temperature, the thermal fuse undergoes melting or fusion to cut off or open the motor circuit. In the disclosed arrangement, however, since the thermal fuse is disposed outside the starter, it is necessary to preclude interference between the thermal fuse and peripheral components of the starter (such as engine accessories, electric wirings and so on). This requirement may deteriorate mountability of the starter with respect to the motor vehicle. The thermal fuse, which is formed as a separate part structurally independent from the motor circuit, increases a number of component parts of the starter and increases the manufacturing cost of the starter.
Starters disclosed in JP-A 10-66311, and FR-A 2785086 include a motor read wire or a brush lead wire (pigtail) having a recessed portion of reduced cross-section, which is fusible to break a motor circuit when heated to a predetermined temperature as an overcurrent passes through the recessed portion. In the disclosed starters, since the recessed portion is formed in a motor lead wire or a brush lead wire of high thermal conductivity, high temperatures generated at the recessed portion is allowed to readily escape therefrom via thermal conduction through the lead wire itself. Accordingly, in order to make sure that melting occurs at the recessed portion, it is necessary reduce the cross-section of the recessed portion to a considerable extent. This requirement, however, increases the risk of a break, which may occur when the locally recessed lead wire is subjected to vibrations during travel of the motor vehicle. Additionally, for a motor circuit in which the lead wire is used, a local reduction in cross section of the lead wire directly leads to an increase in the circuit resistance, which will lower the output of the starter.